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Anita Ashfaqunnesa skips over a ditch oozing with raw sewage, her spotless
white shawl trailing behind her like a superhero's cape, then squeezes
between shacks built on an old rubbish dump. Three years ago, she explains,
this slum in northern Dhaka didn't exist. But with hundreds of thousands of
rural job-seekers pouring into Bangladesh's capital every year, it now teems
with families, and the water they use for drinking, cooking and bathing
comes from pipes that run alongside, and often through, the sewage ditches.
That's why the area's oldest living resident is not a person, but a disease.
"Cholera is a common ordeal here," says Anita, 33. "People don't fear it,
but they are happy to hear there's a vaccine coming to prevent it."

Anita is one of a small army of field workers collecting household data for
the biggest oral cholera vaccination program in history. It starts on Feb.
17 and will involve 240,000 residents in Mirpur, the district that reports
most of Dhaka's cholera cases. Two-thirds of them will receive two oral
doses of a cheap new Indian-made vaccine. "We think of it as a demonstration
project rather than a trial," says Dr. Stephen Luby, Bangladesh country
director for the U.S. Centers for Disease Control (CDC). "We don't have a
big question in our minds over whether this vaccine is going to prevent
cholera. What we're trying to do is illustrate the feasibility of using it
as a public health intervention."
(See TIME's video on the slums of Dhaka.)

Mass vaccination could be a new weapon against an old disease. In Zimbabwe,
where cholera claimed 5,000 lives in 2008 and 2009, a swift vaccination
program could have cut the death toll by 40%, calculated the authors of a
study published in Jan by PLoS Neglected Tropical Diseases. Such results are
avidly followed in Haiti, where cholera has killed about 3,800 people and
sickened 189,000 since October. A committee that includes experts from the
CDC and the World Health Organization (WHO) recently recommended a
small-scale cholera vaccination project. This rankled Haitian health
officials, who want millions to be protected against a disease that foreign
peacekeepers almost certainly brought with them after last year's
earthquake.

Nobody in Bangladesh disputes the origins of the disease. The Ganges delta,
which India and Bangladesh straddle, is cholera's homeland. Six of the seven
pandemics since the 19th century have originated here. Every year, WHO
estimates, there are 3-5 million cholera cases and up to 120,000 deaths
worldwide. Dhaka's dilapidated water and sanitation systems provide ideal
conditions. Bounded by rivers that are too filthy to purify, the city pumps
up nearly all its water from hundreds of deep wells. It is never enough,
especially when those pumps need electricity to run, and Bangladesh is
plagued by power shortages too. With no positive pressure in the water
pipes, sewage and other contaminants easily leak in.

The only thing Dhaka doesn't lack is people. With 13 million residents and
counting, it is a fast-growing megacity in the world's most densely
populated large country. New arrivals squeeze into already overflowing
slums, or squat on wasteland with zero infrastructure. "Wherever there is
human misery you will find cholera," says Dr. Mark Pietroni, Medical
Director of the International Centre for Diarrhoeal Disease Research,
Bangladesh (ICDDR,B) in Dhaka, which is implementing the vaccine project
with the Bangladesh government. "It thrives on malnutrition, overcrowding
and poor hygiene."
(See a brief history of cholera outbreaks.)

Cholera outbreaks in Dhaka are as predictable as the seasons. There are two
each year: roughly one before and one after the monsoon. Dhaka Hospital at
the ICDDR,B treats thousands of cholera patients, who during outbreaks not
only crowd its wards and hallways, but spill out into tents in the parking
lot, forming what might be the world's only hospital ward with speed bumps.
Left untreated, cholera can kill in hours. But treat it promptly and
properly, mainly with oral rehydration salts, and death rates are under 1%.
At Dhaka Hospital, even the sickest patients make near-miraculous
recoveries; arrive with just one breath, say locals, and you'll leave alive.

But as Dhaka's population grows, so does the hospital's patient load. Every
March and April, a thousand new patients a day is standard. "Cholera is a
dreaded illness because of its rapid onset, severity and potential to cause
outbreaks that easily overwhelm public health systems," says Dr. Regina
Rabinovich, director of Infectious Diseases at the Bill & Melinda Gates
Foundation. "That's why it's important to invest in the development of new,
more effective vaccines."

Enter Shanchol, a two-dose vaccine produced by Shantha Biotechnics of
Hyderabad and developed with funding from (among others) the Gates
Foundation, which also gave $16.5 million to the ICDDR,B for the cholera
vaccine project. Shanchol is safe and efficacious: a trial in the Indian
city of Kolkata involving nearly 70,000 people showed that the drug gave 67%
protection for at least 2 years. Just as importantly for mass vaccinations,
it is cheap: its two doses cost about $3, or about a tenth the price of its
only rival, the Dutch-made drug Dukoral. Shanchol is expected to get WHO
approval this year.
(See the top 10 terrible epidemics.)

Bangladesh's state-run immunization programs are widely trusted, so
persuading a cholera-weary populace to take the vaccine shouldn't be hard.
Some 80,000 adults and children will receive it; another 80,000 will receive
the vaccine, plus active encouragement to treat household water and wash
their hands with soap. But assuaging those who don't get it might be
trickier. This includes 80,000 people who will unknowingly receive a
placebo, forming a control group that helps validate the project's results.
"That's what we perceive is going to be our biggest problem: not everybody
gets it," says Luby, who was seconded from the CDC to head the ICDDRB's
Program on Infectious Diseases and Vaccine Sciences.

One of the project's broader aims is to get a better idea of cholera's
mortality rate. "Right now most of the estimates that people throw around
are quite speculative," says Luby. Mortality at the ICDDR,B's hospital may
be less than 1%, but some patients are dead on arrival  negotiating this vast city's gridlocked streets can use up precious hours  and others expire at home.

Mass vaccination has its critics. Today's drugs do not offer long-term
coverage or protect against every cholera strain. And even a cheap vaccine,
in high quantities, is expensive and could divert resources from the only
thing proven to eradicate cholera: improved water and sanitation
infrastructure. (London suffered centuries of cholera epidemics until the
Victorians built sewers.) Improving Dhaka's infrastructure is vital, agrees
Luby, but the task could take decades. The same is true for hundreds of
cities in our rapidly urbanizing world, and indeed for disaster zones such
as Haiti. While that infrastructure is being built or rebuilt, how do you
protect a vulnerable population from cholera? Mass vaccination is one
answer. "What we're trying to do is generate some evidence on what's
feasible and cost-effective," he says.

Participants in the Mirpur project will be monitored for years. But the
vaccine's impact could be felt as early as March or April, when the more
severe of Dhaka's biannual epidemics strikes. "That's why we're aiming to
have this community immunized by the time that worst peak comes," says Luby.
Haiti  and the rest of the world  will be watching.